Frictional materials such as a disc brake pad and a brake lining are used for braking a car and the like. The frictional materials play a role in the brake by frictioning facing members such as a disc rotor and a brake drum. Therefore, for the frictional materials, not only increasing and stabilizing the friction coefficient but also hardly wearing the disc rotor that is a facing member (anti-rotor-abrasion resistance), hardly generating the squeal (squeal properties), and prolonging the life of the pad (abrasion resistance) are required. The durability performance such as no shearing during the braking at a high load (shear strength) or no cracks by the high temperature braking history (anti-crack properties) are also required.
A frictional material includes a binder, a fiber base material, an inorganic filler, and an organic filler, generally in combination with one or two or more kinds thereof to provide the above-mentioned properties. As the fiber base material, an organic fiber, a metal fiber, an inorganic fiber, and the like are used. To improve the anti-crack properties and the abrasion resistance, a copper fiber and a copper alloy fiber are generally used as the metal fiber. To further improve the abrasion resistance, chips and powders of copper and copper alloy may be used. As the frictional material, a non-asbestos frictional material is a mainly used, which is formed by using a large amount of copper, copper alloy, and the like.
However, the frictional material containing copper and copper alloy generates abrasion powder containing copper during the braking. Since it is suggested that the abrasion powder leads to pollution of rivers, lakes, and oceans, there is a movement accelerated to control the use of the non-asbestos frictional material.
To provide a frictional material containing no copper or copper alloy with excellent friction coefficient, abrasion resistance, and rotor abrasion resistance, it is proposed that the frictional material for the brake with a fiber base material, a binder, and a friction adjustment component do not contain a heavy metal or a heavy-metal compound and that the frictional material contains magnesium oxide and graphite in a content of 45-80 vol %, in which the ratio of magnesium oxide to graphite is 1/1-4/1 (see to Patent Literature 1).